1. Field of the Invention
The present invention relates to a disk drive device, and in particular, to a technique for adjusting a reduction in an unbalanced amount in a disk drive device.
2. Description of the Related Art
In recent years, the rotational accuracy of disk drive devices, such as HDDs, has been drastically improved by comprising fluid dynamic bearings as disclosed in, for example, Japanese Patent Application Publication No. 2007-198555. With the improvement, the disk drive devices have been demanded to have higher density and larger capacity. For example, in a disk drive device for recording data magnetically, read/write of data are performed with a magnetic head tracing over recording tracks formed on a recording disk through a slight gap between the two while the recording disk is being rotated at high speed. In order to make such a disk drive device have high density and large capacity, it is needed to make the width of the recording track narrow and to make the gap between the magnetic head and the recording disk further narrower.
Because a recording disk is rotated in a disk drive device, the magnitude of an unbalanced amount in a rotating portion becomes a problem. The unbalanced amount means a deviation amount between the center of gravity of an object and the rotational center of the object when the object is in a motionless state or is being rotated. When an unbalanced amount in a rotating portion of the disk drive device is large, the vibration during the rotation becomes large, thereby it is likely to cause a so-called off-track in which the magnetic head deviates from the normal position, for example, the center of the recording track, when the magnetic head is tracing over the recording track. As stated above, when intending to make the width of the recording track narrow for high density and large capacity of the disk drive device, an influence by the off-track comes to appear remarkably, thereby causing the error rate in reading/writing data to be deteriorated. That is, when an unbalanced amount in the disk drive device becomes large, high density and large capacity of the disk drive device are hampered.
As one of factors by which an unbalanced amount of a disk drive device (hereinafter, the unbalanced amount is referred to as the DBT) is determined, there is the unbalanced amount of a hub itself before a recording disk is installed (hereinafter, the unbalanced amount is referred to as the DBH). In addition, in a state where a recording disk is installed in the outer cylinder portion of the hub, if the gap occurring between the center hole of the recording disk and the outer cylinder portion of the hub deviates to one side, the position of the center of gravity deviates from the rotational center. The deviation occurring when the recording disk is installed becomes one of other factors by which the magnitude of an unbalanced amount is determined. That is, there is an unbalanced amount occurring due to the installation of the recording disk (hereinafter, the unbalanced amount is referred to as the DBD). Accordingly, in accordance with a coincidental combination of the DBH and the DBD, the DBT sometimes becomes large by superimposing the two one on another, or sometimes becomes small by offsetting each other. As a result, a variation in the DBT becomes large, and hence the high density, the large capacity, and the quality stability of disk drive devices have been hampered. Accordingly, it has been tried to make each of the DBH and the DBD small in order to make the DBT small.
As stated above, it can be considered that each of the DBH and the DBD should be made small in order to make the DBT small. For example, as a means for making the DBH small, it can be considered that the part accuracy is improved with each hub being additionally subjected to unbalanced amount adjusting machining in accordance with its unbalanced amount. However, the unbalanced amount adjusting machining is not preferred because of increases in machining time and machining cost, etc. Also, it can be considered that the gap between the center hole of the recording disk and the outer cylinder portion of the hub is made small in order to make the DBD small. However, if the gap is made small, the work efficiency when the recording disk is installed in the hub becomes deteriorated. That is, it is needed to perform assembly work while being careful not to incur deformations or damages of the parts, which incurs deterioration of the work efficiency and hence is not preferred. Also, in this case, it is needed to further improve the machining accuracy, and hence a machining cost is increased. In addition, making the gap between the center hole and the outer cylinder portion small, sometimes becomes an obstacle for the progress in the automatic assembly of disk drive devices.
The present inventors have reached the idea that, if the DBT can be made small almost without machining the hub and the recording disk, such as adjusting machining, and without improving the machining accuracy, the high density and the large capacity of disk drive devices can be attained without hampering an increase in the cost and automatic assembly of the devices.